1. Field of the Invention
The present invention relates to a power resistor suitable as a fixed resistor, a variable resistor, or a resistor array for use in a high-voltage apparatus or in a charger/discharger for a capacitor with a large capacitance, a method of manufacturing this power resistor, and a power circuit breaker including this power resistor as a closing resistor.
2. Description of the Related Art
Generally, the materials of a power resistor are roughly classified into metal resistor materials, metal oxide resistor materials, and nonmetal resistor materials. Of these materials, the metal oxide resistor materials have a high heat resistance, good withstand voltage current properties, and a high energy resistance with which a high electric energy is absorbed, in comparison with those of the other resistor materials.
Representative metal oxide resistors are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 58-139401 and 59-217668.
Jpn. Pat. Appln. KOKAI Publication No. 58-139401 has disclosed a carbon particle dispersed ceramic resistor manufactured by dispersing a conductive carbon powder in an insulating aluminum oxide crystal and sintering the resultant material with clay.
Jpn. Pat. Appln. KOKAI Publication No. 59-217668 has disclosed a carbon-based power resistor which uses, as the raw material, a material formed by adding a carbon powder and a binder to a powder of an insulating inorganic material, such as aluminum oxide, mullite, or calcined clay, and mixing, kneading, and heating the resultant material. Jpn. Pat. Appln. KOKAI Publication No. 59-217668 describes that the carbon powder is a fine powder with a particle size of 0.1 .mu.m or smaller and 1.5 to 5 wt. % of this fine powder are contained.
In the manufacture of a sintered body formed by the addition of a carbon powder to an aluminum oxide powder, the sintering properties of the aluminum oxide are usually impaired. To prevent this, in the manufacture of the carbon particle dispersed ceramic resistor described above the common approach is to add a carbon powder to an aluminum oxide powder and further add clay before sintering in order to make up for the sintering properties of the aluminum oxide. Unfortunately, the addition of clay cannot improve the sintering properties; it merely binds the aluminum oxide power with the carbon powder. The porosity of the resultant resistor, therefore, is as high as 10 to 30%, i.e., the denseness of the resistor is low. As a consequence, the carbon particle dispersed ceramic resistor thus manufactured brings about the following problems.
That is, when this resistor is used as a closing resistor of a breaker which is connected in parallel with a breaking contact for the purpose of absorbing surge occurring upon switching or increasing the breaking capacity, the heat capacity per volume becomes as small as 2 J/cm.sup.3 .multidot.K due to the decrease in the denseness of the resistor mentioned above. Consequently, the temperature of the resistor rises significantly with energy absorption such as surge. Additionally, upon current supply the carbon powder discharges in pores to cause feedthrough discharge. Therefore, a breaker incorporating the carbon particle dispersed ceramic resistor described above is increased in size in order to keep the space for storing the resistor. It is also necessary to keep the breaking capacity low in order to ensure reliability.
Furthermore, since a carbon powder is difficult to uniformly disperse, it is difficult to manufacture a carbon particle dispersed ceramic resistor having the intended resistance with a high reproducibility. Moreover, the resistivity has a distribution in the interior of the sintered body. This brings about a temperature distribution when the temperature of the resistor rises with energy absorption such as surge, with the result that the resistor is broken by the thermal expansion difference. The resistor is also unsatisfactory in strength due to its high porosity.
A power resistor manufacturing method described below is known as the method by which the above conventional problems are solved. That is, this power resistor manufacturing method comprises the steps of: preparing a powder mixture by adding, to a metal oxide powder, a carbon precursor made from an organic compound such as a resol-based phenolic resin and a solvent, and mixing and drying the resultant material; forming a powder compact by molding the powder mixture; and thermally sintering the powder compact in a vacuum or in a non-oxidizing gas at a temperature of 1300.degree. to 1800.degree. C., thereby manufacturing a sintered body containing the metal oxide as a main constituent and 0.005 to 3 wt. % of carbon with a mean particle size of 1 .mu.m or smaller as a sub constituent.
In the above method, the metal oxide powder and the carbon precursor are together dispersed in a liquid phase, mixed, molded, and sintered with heat, and thereby the carbon precursor is converted into carbon by a vapor-phase process. Consequently, it is possible to manufacture a sintered body in which ultrafine carbon is dispersed in the grain boundaries of the metal oxide. Also, in the sintering of the molded product the carbon powder which impairs the sintering properties of the metal oxide powder does not exist. This improves the sintering properties of the metal oxide powder to make it possible to obtain a sintered body with a high denseness. Consequently, a power resistor with a high specific heat per unit volume can be manufactured.
Unfortunately, in manufacturing a sintered body with a high resistivity exceeding 10.sup.3 .OMEGA..multidot.cm by this method, variations in the manufacturing conditions are significantly reflected since the number of carbon connections for imparting conductivity is small. Consequently, it is not necessarily easy to obtain a sintered body having the intended resistivity with a high reproducibility.